Multitechnique Experimental Insight on an Unusual Crystal-to-Crystal High Temperature Solid State Reaction in Zirconium Carboxypyridinephosphonates: From One-Dimensional Chains to Two-Dimensional Hybrid Layers Through HF Elimination

Two novel N-phosphonoethylcarboxypyridines
were
prepared via nucleophilic substitution of two carboxypyridines (namely,
4-carboxypyridine, or isonicotinic acid, and 3-carboxypyridine, or
nicotinic acid) with diethyl-2-bromoethylphosphonate in water. Two
zirconium derivatives of these acids were obtained under mild solvothermal
conditions, and their structures were solved from powder X-ray diffraction
(PXRD) data. The zirconium derivative containing the isonicotinic
moiety (1), with formula ZrF2(HF)­(O3PCH2CH2NC5H4CO2), has a one-dimensional (1D) chain structure in which the carboxypyridine
groups are placed in the external part of the inorganic chain constituted
of zirconium octahedra and phosphonic tetrahedra, whereas the zirconium
derivative containing the nicotinic moiety (2), with
formula ZrF2(O3PCH2CH2NC5H4CO2), has a hybrid layered
structure in which zirconium octhaedra and phosphorus tetrahedra form
a new structural archetype, with a C–O group coordinating the
zirconium atoms. 1 underwent a high temperature (280
°C) slow solid state transformation that involved the loss of
one HF molecule coordinated to the zirconium atom and the replacement
of this coordination vacancy with the neighboring C–O– group belonging to the adjacent chain. The structure of this heated
compound (1a) is a polymorph of 2. A multitechnique
approach, based on coupled Fourier transform infrared spectroscopy
and Raman and solid state NMR spectroscopy allowed us to carry out
a thorough characterization of these materials, finding nice agreements
on the chemical details of this solid state reaction.